Since the Republicans lacked 60 votes in the Senate and zero Democrats supported the Bush tax cuts, The law enacting them was passed using budget reconciliation rules.
> Budget reconciliation is a special parliamentary procedure of the United States Congress set up to expedite the passage of certain federal budget legislation in the Senate. The procedure overrides the Senate's filibuster rules, which may otherwise require a 60-vote supermajority for passage. Bills described as reconciliation bills can pass the Senate by a simple majority of 51 votes or 50 votes plus the vice president's as the tie-breaker
https://www.wikipedia.org/wiki/Reconciliation_(United_States...
Using budget reconciliation rules automatically set a sunset date for the tax cuts, after which they would automatically expire.
When the tax cuts should have automatically expired at the end of 2010, the Democrats controlled the House, Senate and White House. Despite frequent claims over the decade that tax cuts for the rich were morally reprehensible, the Democrats did not allow the tax cuts to automatically expire, extending them for two years in the lame duck session of Congress, and then most were made permanent two years later before they should have expired again.